Cellular communications networks are increasingly being used to support machine to machine (M2M) communication, in which a machine device (MD) communicates with an application server, allowing the server to receive information from the device and to configure the device. Cellular networks may enable such communication between device and application server, regardless of whether or not the server is comprised within the cellular network. It is envisaged that future development of cellular communications networks will include large numbers of such autonomous machine devices, often very small and associated with equipment or apparatus as opposed to a human user. Such devices will typically access the cellular network more or less infrequently, transmitting and receiving very small amounts of data, or being polled for data. In the Third Generation Partnership Project (3GPP) standardisation, machine to machine communication is referred to as Machine Type Communication (MTC), and machine devices are referred to as MTC devices, this being a subset within the larger category of User Equipment devices (UEs). For the purposes of clarity, the following text adopts the 3GPP terminology.
Supporting MTC over a cellular network involves changes to the network architecture, accommodating differences between MTC and user interaction with the network. FIG. 1 shows a reference network architecture 2 used to allow MTC devices to connect to a 3GPP network (UTRAN, E-UTRAN etc). FIG. 1 is reproduced from 3GPP TS 23.682 V11.3.0, “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Architecture enhancements to facilitate communications with packet data networks and applications (Release 11)”, December 2012. Additional shading of elements relating to MTC devices has been introduced to the Figure. Referring to FIG. 1, the UE, in this case an MTC device, can be seen to include an MTC UE application 6. The MTC device communicates over the radio access network (RAN) to access application servers via the core network of its home public land mobile network (HPLMN). Within the HPLMN various network entities may be introduced according to the particular model envisaged for MTC traffic. FIG. 1 illustrates a direct model, in which an application server (AS) 8 connects directly to a 3GPP operator network to perform direct communications with the MTC device. FIG. 1 also illustrates an indirect model, in which an application server (AS) 10 connects indirectly to an operator network via a Services Capability Server (SCS) 12. An MTC Inter Working Function (MTC-IWF) 14, acts as an interface enabling interworking of the 3GPP core network and the MTC service capability. A hybrid model may use both direct and indirect models simultaneously. The network architecture may also include an MTC Authentication, Authorisation and Accounting (AAA) function 16 within the HPLMN.
The nature of MTC devices and their envisaged use patterns mean that these devices are often required to be highly energy efficient. External power sources for MTC devices will often not be available, meaning the device must operate using energy harvesting or battery power, with frequent replacing or recharging of batteries being neither practical nor economically feasible. Efforts to increase the energy efficiency of MTC devices have generally focused on increasing the efficiency of operation of such devices within cellular networks such as Evolved Packet System (EPS) networks.
Many such methods seek to enable and enhance the possibility for MTC devices to spend time in energy efficient low power modes between communication events. One way of achieving this is to extend the maximum possible DRX cycle length in both idle and connected modes for EPS/LTE (Evolved Packet System/Long Term Evolution) networks. Another possibility is to allow the MTC device to detach from the network after each communication event, and reattach when a need arises, for example when the device has uplink data to send or when it wants to poll a server for data or at predefined intervals agreed with the network. This “detach-reattach” method allows the MTC device to enter the “deepest sleep”, or most low power state, with a large amount of the electrical circuitry in the device turned off. This deep sleep offers considerable advantages in energy saving. However, the network attach procedure is associated with a significant signalling overhead, and the need to perform such a procedure before each communication event reduces the overall energy efficiency of the detach-reattach method, owing both to the signalling overhead itself and to the time spent in non sleep modes while the attach or detach procedures are carried out.